voltammetry
TRANSCRIPT
Voltammetry and polarography1. NORMAL PULSE VOLTAMMETRY
2. DIFFERENTIAL PULSE VOLTAMMETRY
By Shobana.N.SQueen Mary’s College, Chennai
Voltammetry
It is an Electro- analytical technique.
It gives information about the analyte.
We can even measure the amount of current by varying the voltage.
The change in current with the varying voltage gives the plot and is known as uoltammogram
There is a minimum potential required to initiate an oxidation or reduction reaction at an electrode.
Instrumentation
It is a three electrode system.
1. Working electrode ; 2. Reference electrode and
3. Auxiliary electrode.
(1) working electrode;
(2) auxiliary electrode;
(3) reference electrode
Working Electrode
Ranges from small mercury drop to flat platinum disc.
Platinum disc
DME
Other commonly used electrode materials gold, platinum and glassy carbon.
Glassy carbon Gold electrode
Depending on the choice of working electrode, the type of voltammetry is decided.
Example :
We use Dropping Mercury Electrode (DME) in Polarography technique
We use Platinum electrode in Cyclic Voltammetry
We use Glassy Carbon as electrode in Linear Sweep Voltammetry.
Reference Electrode
Usually Standard electrode is used as Reference electrode.
Its potential is constant.
It provides potential to the Working electrode.
Common reference electrode are :
Calomel electrode and Ag/AgCl electrode
Auxiliary Electrode
It is usually a thin platinum wire.
It serves merely to carry the current flowing through the cell.
Usually redox reaction occur simultaneously at the
auxiliary electrode.
Type of Voltammetry
LINEAR SWEEP VOLTAMMETRY
STAIRCASE VOLTAMMETRY
CYCLIC VOLTAMMETRY
SQUAREWAVE VOLTAMMETRY
ANODIC STRIPPING VOLTAMMETRY
CATHODIC STRIPPING VOLTAMMETRY
ABSORPTIVE STRIPPING VOLTAMMETRY
ALTERNATING CURRENT VOLTAMMETRY
POLAROGRAPHY
ROTATED ELECTRODE VOLTAMMETRY
NORMAL PULSE VOLTAMMETRY
DIFFERENTIAL PULSE VOLTAMMETRY
CHRONOAMPEROMETRY.
Linear Sweep Voltammetry
In linear sweep voltammetry (LSV) a fixed potential range.
the voltage is scanned from a lower limit to an upper limit.
In LSV measurements the current response is plotted as a function of voltage rather than time.
The scan begins from the left hand side of the current/voltage plot where no current flows.
As the voltage is swept further to the right (to more reductive values) a current begins to flow and eventually reaches a peak before dropping
Normal Pulse Voltammetry
Normal polarography has been replaced by various forms of pulse polarography.
It uses a series of potential pulses.
Here the pulse time (tp) is 50ms.
Each potential has different amplitude
Differential Pulse Voltammetry
If potential pulse is applied periodically to the Linear Sweep Voltammetry, then it is known as Differential Pulse Polarography.
Hence it is denoted as the differential of linear sweep voltammetry.
The current is measured twice per cycle.
Here the pulse time (tp) is 17ms.
The difference in the two currents gives rise to the peak-shaped voltammogram.
consistent enhancement of the signal is achieved.
Detection limit is as low as 10-8 M
Each potential step has the same amplitude
Polarography
Polarography is the branch of voltammetry in which a dropping mercury electrode (DME) is used as the working electrode.
Conventional DC
Wide cathodic potential range and a renewable surface
Hence widely used for the determination of many reducible species
Reduction begins at sufficiently negative potential
voltage applied to the polarisable electrode is increased over the range of 0-2V
It is known as incomplete electrolysis.
Jaroslav Heyrovsky
He was the inventor of Polarographic technique.
He received Nobel Prize in 1959
He is known as father of electro analytical
method.
Polarography has a special characteristic :
1. Use of polarized electrode.
2. Use of depolarized electrode.
POLARIZED ELECTRODE : DME
DEPOLARIZED ELECTRODE : SCE
Dropping Mercury Electrode
It is the working electrode in one of the voltammetry technique known as polarography
Advantages of DME
Clean Surface generated
Rapid constant current achievement
Even metal with high negative standard voltage systems can be studied without hydrogen.
Amalgam formation more favourable.
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